Correct spelling, explanation: twelfth is an ordinal number from twelve. In twelve there is a consonant v, so it may seem, that after adding the ordinal suffix -th, the form would be twelvth. However, we can’t do it due to pronunciation. T in the suffix is an unvoiced consonant, so a consonant directly before must be unvoiced as well. F and v are technically the same sound (the former is unvoiced and the latter is voiced), so they are changed with pronunciation. This is why we spell this word twelfth.

Definition of twelfth:
numeral, the ordinal number between eleventh and thirteenth.
It’s Julie’s twelfth birthday next week.
We were twelfth in line for the restaurant yesterday, I thought we would never sit, but it went smoothly.

Incorrect spelling, explanation: twelvth is the wrong form because v is a voiced consonant and t is an unvoiced one, so they can’t occur one after another. Because of the fact, that v and f are linguistically speaking the same sound (but f is unvoiced), for better pronunciation it was changed in twelfth. The same happens for example in five and fifth. Therefore twelfth is the correct spelling.